Optional V3 Web Search and support for search SLURLs

Added "Use web-based search instead of legacy v1 search" checkbox to System->General preferences

Unified all functions to display search floater into toggle_search_floater and LLFloaterSearch::showInstance
Added UseWebSearch and UseWebSearchSLURL debug settings for flexible configurations
- SLURLs with legacy search cannot always present the same results as web
Added SearchURL debug setting, a separate setting from the one used in the all tab, slightly different and always set by response["search"] on OSGrid logins to detect if websearch is supported
Cleaned up llfloaterdirectory includes and added "Search.WebFloater" to the CommitCallbackRegistrar in case we ever want to add a button or something to open web search (couldn't find a nice place for it, sadly), also added more functions to show specific tabs and performQueryOn to search a tab with specific query on LLPanelDirBrowser's

indra/newview/llpaneldirevents.cpp

@@ -215,7 +215,7 @@ void LLPanelDirEvents::performQueryOrDelete(U32 event_id)
 	U32 cat_id = childGetValue("category combo").asInteger();
 
 	params << cat_id << "|";
-	params << childGetValue("event_search_text").asString();
+	params << childGetValue("name").asString();
 
 	// send the message
 	if (0 == event_id)